Idiomatically creating a multi-value Map from a Stream in Java 8

Question

Is there any way to elegantly initialize and populate a multi-value Map> using Java 8\'s stream API?

I know it\'s possible to cr

Answer 1

If you use forEach, it’s much simpler to use computeIfAbsent instead of compute:

Map<String, Set<Person>> personsByName = new HashMap<>();
persons.forEach(person ->
    personsByName.computeIfAbsent(person.getName(), key -> new HashSet<>()).add(person));

However, when using the Stream API, it’s preferable to use collect. In this case, use groupingBy instead of toMap:

Map<String, Set<Person>> personsByName =
    persons.collect(Collectors.groupingBy(Person::getName, Collectors.toSet());